Wind energy is increasingly recognized as a viable option for complementing and even replacing other types of energy sources such as for instance fossil fuels. However, the control of the operation of a wind turbine is often very sensitive because a wind turbine is typically exposed to ever-changing ambient conditions which may cause extreme mechanical loads for instance to the rotor and to the mast of the wind turbine.
Further, the operation of a wind turbine strongly depends on the availability and the electrical power absorption capability of the electricity network the wind turbine is connected to. Specifically, when an electricity network fault happens near a wind turbine, the voltage of the electricity network will be typically very low. This makes it impossible for the generator of the wind turbine to export the produced electrical power to the electricity network. The result is an increasing of the rotational speed of the rotor of the wind turbine with the risk of a dangerous over speed situation.
In order to avoid an over speed situation it is known to control the rotational speed of a speed controller of the wind turbine for instance by setting the pitch angle of the blades of the rotor to an angular position being different from the optimal blade pitch angle when the wind turbine is operated in an error-free operational state. Thereby, it is essential that the speed controller is able to react quickly on the electricity network fault. Further, an emergency stop of the wind turbines operation can be necessary due to the fact that without voltage on the electricity network, a blade pitch angle control system can only be operated in the emergency mode, where it is changing the blade pitch angle with a constant speed. However, pitching with a constant speed causes large loads on the tower and yaw system of the wind turbine.
EP 1 651 865 B1 describes a method for regulating the pitch of blades of a wind turbine on detection of a malfunction of the electricity network. Thereby, the pitch is regulated continuously or in steps in order to reduce the power production of the wind turbine and thus protect the components of the wind turbine from overheating.
EP 1 819 023 A2 solves the problem of overheating components of a wind turbine in case of an electricity network malfunction by switching the power output from the turbine to a dump load as soon as the malfunction has been rectified or until the wind turbine has been stopped in a controlled manner.
There may be a need for providing a control for the rotational speed of a wind turbine which is impeded to export electrical power to an electricity network, wherein the control changes a characteristic operational parameter being indicative for the rotational speed of the rotor in such a manner that mechanical loads on the tower and yaw system of the wind turbine can be reduced.